1. Field of the Invention
The present invention relates to a stencil mask for ion plantation used in an Ion Implantation process of semiconductor device fabrication.
2. Related Art
In semiconductor device fabrication, it is important to control a conductivity type and an impurity profile of a silicon single crystal substrate by introducing impurity atoms of III and V families therein. As to methods of introducing impurity atoms, a diffusion method and an ion implantation method are given for examples.
The diffusion method is to thermally diffuse desired atoms in crystal lattices of a substrate. Generally, a glass layer including impurities is formed on a substrate surface, and by a substitution due to an oxidation-reduction reaction between the glass layer and atoms in the substrate, impurity atoms are introduced into the substrate.
However, recently, in response to higher integration of LSI, further precise control of impurities (such as a junction depth and decrease in variation of resistance) has been required, and in response to mass production, improvements of repeatability and processing capacity have also been required.
Accordingly, the importance of the ion implantation method, which is the other example of impurity control methods, has been increased, and the ion implantation method has been used practically as a substitute for a conventional diffusion method. The principle of the method is that ions having high energy are collided with a substrate to bury them physically, and defects are recovered and impurities are activated as carriers by a subsequent heat treatment, When the substrate is made of silicon, ions such as B in the III family, or P, As, or Sb in the V family, for example, are implanted as impurities.
This ion implantation method is basically characterized in that masking for controlling an implantation region is possible. If a material of the mask sufficiently thicker than the range of ions to be implanted is used, the ions stop inside the mask and do not reach into the substrate under the mask. As for such a mask for ion implantation, conventionally, a method using a resist mask having a necessary thickness and formed on a desired region of a substrate by a photolithography process has been employed. However, as for a conventional resist mask produced by a photolithography, there is needed a long process in which a resist mask is formed by applying resist, exposing and developing it, and after the mask is used in an ion implantation process of semiconductor device fabrication, the resist mask no longer required is subjected to ashing for removal and cleaned in conclusion. Therefore, the reduction of processing hours has been required.
Under such a circumstance, recently, an ion implantation technique using a stencil mask for ion implantation was suggested (see “T. Shibata, et al., “Stencil Mask Ion Implantation Technology for High Performance MOSFETs”, IEDM 2000 Proceedings, San Francisco, Calif. (Dec. 11–13, 2000)”). It was reported that in the ion implantation technique, shorter process time, lower manufacturing cost, and decrease in an occupied area of an apparatus to be required are achieved.
Conventional stencil masks for ion implantation have been produced from easily obtainable SOI (Silicon on Insulator) substrates, and each stencil portion is made of a silicon single crystal material. For example, the stencil mask for ion implantation described in the aforementioned reference (T. Shibata, et al.) has a structure that a stencil portion made of silicon and having a thickness of 5–10 μm in which a patter is formed is supported by a base material portion made of silicon and having a thickness of 500 μm. However, when the stencil portion is made of silicon as described above, its lifetime in practical use has come to an issue.
To be more precise, there are problems in that (1) since stress of the stencil portion made of silicon is changed by ion irradiation, its implantation precision is decreased, and (2) since bond of silicon in a stencil mask is broken, silicon atoms are implanted as impurities.